Ethernet as a Control Network


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Ethernet as a Control Network

  1. 1. Ethernet as a Control Network INDCOM 2003 29 April to 1 May
  2. 2. Introduction Ethernet’s worldwide acceptance in industrial and office environments has created an eagerness to expand its responsibilities on the plant floor. Ethernet is widely used for information (office, Human Machine Interface (HMI), controller programming, etc.) communications today. The network’s performance capabilities make it ideal for tasks such as data monitoring and program maintenance. However, many predict that recent technological advancements in Ethernet, and the emerging Fast Ethernet technology, will also enable it to handle mission-critical control responsibilities currently being managed by existing industrial automation networks. Meanwhile, others contend that Ethernet has a long way to go before it can assume an expanded role in the manufacturing environment. Ethernet technology brings availability, familiarity, and possible cost benefits. Until recently, however, creating an industrial control system by using Ethernet at the device (I/O) network was not feasible due to a number of factors, including Ethernets lack of determinism, the need for interoperability among devices, security concerns, etc. Determinism is the ability to predict when information will be delivered. To guarantee this, an industrial control network must provide scheduled bandwidth (or time slots) that are reserved for time-critical data transfer. Communication over Ethernet, however, is based upon collision detection. If a device attempts to send a message, and that message collides with another message on the Ethernet media, the device backs off and waits to transmit. Thus cannot guarantee determinism. Recent advances in switch technology have now enabled Ethernet to approach determinism. Switches, unlike traditional bridges and hubs, reduce traffic between the devices attached to their ports. Moreover, the IEEE 802.3 Standard provides for standardized full-duplex operation, which gives a single node - in a point-to-point connection to the switch - full wire concentration. As a result, full- duplex switched Ethernet networks are theoretically able to avoid collisions. The requirement for device interoperability, the ability of products from different control vendors to communicate with each other, has been answered by a plethora of industrial communication protocol ‘standards’ from a wide community of automation vendor groups. Ethernet as a Control Network Page 2 of 16 INDCOMM 2003
  3. 3. Ethernet OSI Model All installed Ethernet networks support one or more communication protocols that run on top of Ethernet and provide sophisticated data transfer and network management functionality. The communications protocol determines what level of functionality is supported by the network, what types of devices may be connected to the network, and how devices interoperate on the network. TCP/IP (transmission control protocol/internet protocol) is the communications protocol of the Internet. TCP/IP is a layered protocol that can be mapped approximately to the OSI (Open System Interconnection) seven-layer network model shown in the following figure. The OSI model represents the components of a standard open network architecture. In this model, Ethernet represents Layers 1 (Physical) and 2 (Data Link). The Internet Protocol (IP) maps to Layer 3 (Network). The TCP and UDP transports map to Layer 4 (Transport). The TCP/IP protocol suite has no specific mapping to Layers 5 and 6 of the model. The user services commonly associated with TCP/IP networks map to Layer 7 (Application). Each layer of the OSI model uses the services provided by the layer immediately below it. For example, when a TCP connection needs to send a packet of data to another device over Ethernet, it passes the packet to IP for transmission. IP then handles the interface to Ethernet and ensures that the packet gets transmitted onto the Ethernet network to the destination device. On the receiving end, the IP layer receives the packet from the Ethernet interface, and passes it to the appropriate TCP connection within the receiver. Ethernet as a Control Network Page 3 of 16 INDCOMM 2003
  4. 4. Physical Layer The topology, or physical configuration, of the original Ethernet networks was primarily a multi-drop bus topology. With a bus topology every device on the network can send data at any (same) time. All devices share the same logical medium. As more devices are added to the network, bus contention increases. In addition, bus-based designs are not indefinitely expandable due to increased propagation delay when bus length is increased. The emergence of repeater hubs and active switches enabled Ethernet networks to be configured in a star topology, where the hub or switch acts as a network concentrator for connecting multiple devices or Ethernet network segments. Ethernet as a Control Network Page 4 of 16 INDCOMM 2003
  5. 5. This is the most common topology found in new Ethernet installations today. The hub/switch and its attached devices and segments may comprise the entire Ethernet network, as would be typical in a small office environment. Or, the hub/switch may be linked to another hub/switch, or to a fiber-optic backbone that spans a building or campus, with hubs or switches connected to the backbone at various points using uplink ports. The original Ethernet specifications described a physical network layer running at 10 Mbit/sec. More recent developments in Ethernet technology include Fast Ethernet (running at 100 Mbps/second) and Gigabit Ethernet. Data Link Layer The Data Link Layer uses CSMA/CD (carrier sense multiple access/collision detection) to manage bus contention for the network. A collision occurs when two or more devices attempt to transmit at the same time. Each of the colliding devices must then backoff and attempt to gain access to the wire according to the CSMA/CD arbitration mechanism. Note that a collision is not an event to be avoided, but simply a mechanism to allocate shared bandwidth for stations which want to send data at the same time. The resolution occurs very quickly. The station almost immediately aborts the transmission, gets off the channel, and retransmits the frame after a random backoff time. Very little channel time is wasted for the backoff as compared to valid data transmission times. The first range of backoff time is 0...51.2us. An increase of the number of collisions on an Ethernet is therefore not necessarily indicative of a problem, but only an indication that there is more offered load. Because it is impossible to predict the amount of time required for all colliding devices to successfully complete their message transmission, the CSMA/CD mechanism and its performance consequences has earned Ethernet its reputation for being non-deterministic. However, depending upon the used bandwidth of the network, data updates are still processed in a fast (milliseconds) time frame. For example, an update may occur after 20ms, the next after 26ms, the next after 23ms, etc., instead of exactly every 25ms. And if the updates are required only every 50ms, the network is effectively deterministic for this application. Faster Ethernet The use of Fast Ethernet can also provide a noticeable improvement over 10 Mbit Ethernet in the area of collision recovery. The backoff times for 100 Mbit Ethernet are 1/10th of those for 10 Mbit Ethernet. On a loaded network where collisions are an issue, 100 Mbit Ethernet will show noticeably better performance than 10 Mbit Ethernet. In addition, a 100 Mbit Ethernet network is able to handle a larger offered load than a 10 Mbit Ethernet network before collisions become an issue. Coupled with Star topologies and the use of full duplex active switches most of the collisions on an Ethernet Network can be eliminated. Ethernet as a Control Network Page 5 of 16 INDCOMM 2003
  6. 6. Gigabit Ethernet is an emerging technology defined in IEEE specification 802.3z. It is basically Ethernet operating at 1000 Mbits/second (1Gigabit/sec). It is 100 times as fast as the original Ethernet, and 10 times as fast as Fast Ethernet. Like those technologies, it uses the frame format, addressing scheme and CSMA/CD mechanism described in the original 802.3 specifications. Gigabit Ethernet is Ethernet. It is designed to run over both fiber and copper media. Gigabit Ethernet at this time is primarily targeted for use as an enterprise-wide backbone. It is likely that for at least the near future, the cost of this technology will preclude its use down to the level of individual workstations, printers and other Ethernet end-node devices. Network Layer Ethernet provides only the Physical and (Data) Link layers seen at the bottom of the OSI model. For this reason, all Ethernet networks support upper layer protocols that run on top of it, providing sophisticated data transfer and network management functionality. The Network layer provides the internetworking protocol for the communications session. IP IP (Internet Protocol) provides the routing mechanism. TCP/IP is a routable protocol, which means that all messages contain not only the address of the destination station, but the address of a destination network. This allows TCP/IP messages to be sent to multiple networks within an organization or around the world, hence its use in the business world and in the worldwide Internet. Every client and server in a TCP/IP network requires an IP address, which is either permanently assigned or dynamically assigned at startup. Transport Layer The transports supported by the TCP/IP protocol suite are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). They both map to the Transport Layer of the OSI model. TCP TCP is a connection-oriented transport that provides reliable transmission of data from one device to another. Once a TCP connection is established between two devices, TCP handles fragmentation and re-assembly of message packets, detects failures, performs retries, and generally provides a high quality of service between the two devices. TCP guarantees the data will get from one device to the other if it is possible. If the transmission fails for any reason, TCP ensures that the applications on both ends of the TCP connection know it. TCP presents data to the application layer above it in the form of a continuous byte stream. The receiving application must be capable of recognizing any message delimiters that might be embedded in the byte stream by the transmitting application. TCP works only in unicast (point-to-point) mode, and is used by applications such as FTP (File Transfer Protocol), HTTP (Web Server), and Telnet (terminal Ethernet as a Control Network Page 6 of 16 INDCOMM 2003
  7. 7. emulation). In an industrial automation application, TCP would typically be used to download ladder programs between a workstation and a controller, for HMI devices that read or write controller data tables, or for peer-to-peer messaging between two controllers. UDP UDP is a much simpler transport protocol. It is connectionless and provides a very simple capability to send datagrams between two devices. UDP is used by applications that implement their own handshaking between two devices and only want a minimal transport service. UDP is smaller, simpler, and faster than TCP due to its minimal capabilities and use of resources. UDP can operate in unicast, multicast or broadcast mode. In an industrial automation application, UDP would typically be used for network management functions, applications that do not require reliable data transmission or applications that are willing to implement their own reliability scheme, such as flash memory programming of network devices. Application Layer In order to provide interoperability among devices a common Application layer is needed. It is this upper layer’s protocols that determines the level of functionality a network supports, which devices may connect to the network, and how devices interoperate on the network. Ethernet can only be as efficient as the network whose upper-level protocols it uses. The TCP protocol suite provides a set of services that two devices use to share data. However, TCP does not guarantee these devices can communicate effectively, if at all. It only guarantees that messages can be transferred between the two devices. A common language is still needed for communication. For an industrial version of Ethernet that language is a universal Application layer. Ethernet as a Control Network Page 7 of 16 INDCOMM 2003
  8. 8. Example of an Industrial Automation (Control) Protocol EtherNet/IP is an industrialized extension of Ethernet TCP/IP which uses an approach called “TCP/IP encapsulation” to apply a common application layer over Ethernet. TCP/IP encapsulation allows a device node to encapsulate a message as the data portion in an Ethernet message. The node then sends the message - TCP/IP protocol with the message inside - to an Ethernet communication chip (the Link layer). The standard application layer makes interoperability and interchangeability of industrial automation and control devices on Ethernet a reality for automation applications. EtherNet/IP uses TCP/IP to send explicit messages - those in which the data field carries both protocol information and instructions for service performance. With explicit messaging, nodes must interpret each message, execute the requested task, and generate responses. These types of messages are used for device configuration and data collection, and are highly variable in both size and frequency. In the industrial environment, TCP/IP is typically used to download ladder programs between a workstation and a controller, for HMI devices that read or write controller data tables, or for peer-to-peer messaging between two controllers. For control (real-time) messaging, EtherNet/IP employs the User Datagram Protocol/Internet Protocol (UDP/IP), which can multicast (i.e., broadcast) and send implicit (real time) messages. The data field contains no protocol information, only real-time I/O data. The application layer CIP takes care of monitoring the UDP packet. The meaning of the data is predefined at the time the connection is established, and therefore processing time in the node is minimized during runtime. UDP messages are low overhead, short, and provide the required, time-critical performance needed for control. By using both TCP/IP and UDP/IP protocols to encapsulate networked messages, both real-time I/O and explicit messaging can occur. EtherNet/IP provides Ethernet users with real-time I/O, device-configuration, and diagnostic capabilities, along with interoperability and interchangeability. The Application layer used by EtherNet/IP (called CIP protocol) provides the control functionality to TCP, IP and UDP. CIP handles handshaking at the application level. It supports a common object library, device profiles, control services, and routing. These objects and profiles make it possible for plug-and- play interoperability among complex devices from multiple vendors. The object definitions are rigorous and support real-time I/O control, configuration, and data collection over the same network. Ethernet as a Control Network Page 8 of 16 INDCOMM 2003
  9. 9. CIP uses the “producer/consumer” (also called publish and subscribe) networking model, replacing the old source/destination (master/slave) model. The producer/consumer model contains all source/destination capabilities plus additional capabilities for improved efficiency. In the source/destination model, the source communicates with each destination, one at a time. Real time data must be adjusted to maintain accuracy as communication takes place with each source, one at a time. Some of the destinations may not need the information, so that effort is wasted. Moreover, the delivery time changes with the number of destination devices. In the producer/consumer model one producer broadcasts (multicasts) the data once to all the consumers. All consumers see the data simultaneously, and may choose whether to consume (receive) the data or not. Delivery time is consistent and bandwidth usage is optimized, no matter how many consumers there are. Bandwidth optimization is especially important on Ethernet, where the amount of data on the wire determines the number of collisions. In addition to multicast data consumption, the producer/consumer model provides for change-of-state and cyclic I/O. With change-of-state (event-driven), a sensor produces data only when the object is present. Cyclic (time-driven) I/O can be used for scheduled data transmission. Ethernet as a Control Network Page 9 of 16 INDCOMM 2003
  10. 10. Evolution of Switches Bridges and Routers The original Ethernet topology was a multi-drop bus architecture. With this architecture, bridges and routers are used to reduce transmission time and increase overall performance. Bridges are multi-port devices that connect network segments that use different physical media. Bridges also monitor network traffic, building and maintaining internal tables that list the port on which each Ethernet address resides. When a bridge receives a packet destined for a particular address, the bridge retransmits the packet only on the port at which the device resides. Each port on a bridge represents a separate collision domain. A router is a device that forwards traffic between networks based on network layer information in the data and on routing tables it maintains. The router builds up a logical picture of the overall network in its routing tables, and then uses this information to choose the best path for forwarding network traffic. Bridges and routers have similar bus-based architectures that function on shared media. Data is received into a buffer and examined prior to forwarding. Multiple segment contention is necessary for access to the bus. Bridges and routers also have relatively high latencies (the time between initiating a request for data and the beginning of the actual data transfer). Hubs A hub (also called a “repeater hub”) is a common wiring point for star-topology networks. Hubs have multiple ports to attach the different cable runs. Some hubs include electronics to regenerate and retime the signal between each hub port. Others act as signal splitters, similar to the multi-tap cable-TV splitters you might use on your home antenna coax. Some reroute the network signals to each active device in series, other hubs redistribute received signals out all ports simultaneously. However, all devices connected to a hub reside in the same collision domain, meaning that their transmission behavior is governed by the CSMA/CD mechanism to resolve contention for the use of the wire. This precludes determinism and makes hubs impractical for use in real time control systems. Switches In recent years hub technology has been supplanted by a newer high speed switching techniques to allow traffic between any two ports on the switch to pass through the switch with an extremely low latency in the order of microseconds. This technology has been enabled by specialized hardware that can support a very high bandwidth backplane within the device. The speed of the backplane is typically greater than the sum of the speeds of the Ethernet ports on the device, and can accommodate all of the ports running at full speed without collisions. Furthermore, these devices are capable of buffering frames temporarily to handle short-term contention for the same output port. Ethernet as a Control Network Page 10 of 16 INDCOMM 2003
  11. 11. Switches are descendents of bridges. Like traditional bridges, switches build and maintain internal tables that map Ethernet addresses to ports. A packet received on one port is rapidly “switched” to the appropriate output port. Each port on the switch is its own collision domain, so collisions between devices attached to the switch do not occur. Switches eliminate the bus architecture. A switch segments a network into many parallel dedicated lines to produce a contentionless, scalable architecture. The switch establishes a direct line of communication between two ports and maintains multiple simultaneous links between various ports. The switch uses the addressing information in each Ethernet frame to forward data only to the port connected to the destination device. The switch manages network traffic by reducing media sharing since traffic is directed only to the segment for which it is destined. Switching Methods There are two basic methods of switching: Cut-through switching starts sending packets as soon as they enter a switch and their destination address is read. The entire frame is not received before a switch begins forwarding it to the destination port. This reduces transmission latency between ports, but it can propagate bad packets and broadcast storms to the destination port. Store-and-forward switching buffers incoming packets in memory until they are fully received and a cyclic redundancy check (CRC) is run. This reduces bad packets and collisions that can adversely effect the overall performance of the segment. However, the buffering adds latency to the processing time. The latency increases in proportion to the frame size. Some switches perform on both levels. They begin with cut-through switching, and monitor the number of errors that occur. When that number reaches a certain threshold point, they become store-and-forward switches. They remain so until the number of errors declines, then they change back to cut-through. This is known as threshold detection or adaptive switching. Full Duplex Switch Operation The use of active switches in full duplex mode further increases the determinism of an Ethernet network. By sending and receiving information at the same time, a full duplex 10 Mbit network effectively operates at 20 Mbit. A 100 Mbit network effectively operates at 200 Mbit. These very high speed transmission rates virtually makes concerns about Ethernet’s lack of determinism go away. Ethernet as a Control Network Page 11 of 16 INDCOMM 2003
  12. 12. VLAN Advanced switches support a virtual LAN (VLAN) feature that allows users to configure the switch so that ports are subdivided into groups such that all packets received on one port will be transmitted on a specified group of ports. The receiving port and the group of transmitting ports constitute a VLAN. VLAN’s may typically be overlapped within a switch, such that any one port may appear on multiple VLAN’s. This feature allows the user a great deal of flexibility over partitioning the ports on a switch into multiple overlapping collision domains. IGMP Snooping IGMP snooping constrains the flooding of multicast traffic by dynamically configuring switch ports so that multicast traffic is forwarded only to ports associated with a particular IP multicast group. Switches that support IGMP snooping "learn" which ports have devices that are part of a particular multicast group and only forward the multicast packets to the ports that are part of the multicast group. Performance Limitations of Switches Switches do have some performance limitations that may affect some applications. If a switch experiences internal congestion due to message packets on multiple input ports contending for transmission to the same output port, the switch may drop packets, or it may force a collision back to the transmitting devices so they back off long enough for the congestion to clear. The approach taken depends upon the implementation chosen by the switch vendor. In either case, a variable latency is inserted into the message stream. This is generally not a problem for office applications, but may have profound impact on industrial automation applications. Cost Ethernet media components are based on the IEEE 802.3 standard, which is available to the public for a small fee. The open nature of Ethernet and its phenomenal growth has encouraged many companies to enter the market and build Ethernet media components such as hubs, switches, cables, connectors, and assorted network monitoring and maintenance tools. This competition in turn has placed downward pressure on the cost of “off-the-shelf” Ethernet technology to the end user. However, for networked devices in an Industrial Automation application, the cost of Ethernet connectivity is influenced by additional factors. Industrial network products need to be built to withstand higher ranges of temperature, humidity and electrical interference than most typical commercial “off-the-shelf” Ethernet products were ever designed to handle. They should have been designed and tested for compliance to the rigorous environmental standards typical for industrial control devices (industrial CE mark, shock, vibration, etc.). Ethernet as a Control Network Page 12 of 16 INDCOMM 2003
  13. 13. Additionally, industrial Ethernet device interfaces usually include not just the Ethernet interface hardware (transceiver, controller), but also include a high- speed microprocessor, substantial RAM memory, ROM memory (EPROM or Flash), and other components required to support the application interface. An operating system, TCP/IP protocol stack and application software are typically embedded in the ROM memory on the interface. The embedded software handles all Ethernet-TCP/IP communications as well as interfacing to the automation application. The strict environmental qualifications, complex hardware and embedded firmware result in a communications interface that is more complex and expensive than a typical off-the-shelf $50 Ethernet Network Interface card. Advantages of Ethernet for Control Wide Acceptance – Ethernet is an established, worldwide standard with support from IEEE and the International Standards Organization. In addition to this support from standards organizations, Ethernet has been broadly used in both industrial and office environments. The high number of users has, in turn, ensured the downward price of Ethernet components. Plus, IS and IT departments worldwide have been using Ethernet for years. Such long-term exposure to the Ethernet technology has produced an expansive knowledge base and unparalleled resources. Speed – Recent developments in Ethernet technology include Fast Ethernet and Gigabit Ethernet. Fast Ethernet (100 Mbps/sec) provides a wire speed that is 10 times as fast as traditional Ethernet, which benefits bandwidth-hungry applications, as well as the transfer of large data files over the network. Gigabit Ethernet is an emerging technology that is basically Ethernet operating at 1000 Mbps/sec. Integration with Internet/Intranet – All installed Ethernet networks support one or more communications protocols that run on top of Ethernet and provide sophisticated data transfer and network management functionality. Of these, TCP/IP is receiving the most attention due to the global Internet (including the World Wide Web) and the corporate Intranets that are transforming how corporations distribute information today. Many believe that using Ethernet (especially if the organization dabbles in e-commerce) at all levels in the factory will help integrate and optimize the flow of information from the shop floor to the Intra/Internet. Broadcast/Multi-cast Traffic - I/O traffic will not typically pass through a router. By design, the Time-To-Live parameter (see Internet Protocol for details of TTL parameter) is configured for a value of 1. This value will be decremented by any router and then discarded. A value of 1 is selected to avoid attempts to implement I/O control (high-speed) through a slow network device (router) or through a slow network. Ethernet as a Control Network Page 13 of 16 INDCOMM 2003
  14. 14. Concerns Broadcast/Multi-cast Traffic - Although switches isolate separate collision domains on each port, they do not create separate broadcast domains. However, each VLAN is a separate broadcast domain, if this feature is enabled on the switch. An Ethernet broadcast message that is received on any port will be re-transmitted on all switch ports, to all attached devices. This means that switches do not eliminate the problem of excessive broadcast traffic that can cause severe performance degradation across an entire Ethernet network when a damaged or improperly configured device is attached to the network. Some switch vendors are working on proprietary methods for suppressing excessive broadcast messages in their switches, but this is not universal. Broadcast messages are common on Ethernet networks that carry the TCP/IP protocol because Ethernet broadcast messages are used by TCP/IP for address resolution. However, broadcast traffic represents a small percentage of network traffic on a network that is properly configured and operating normally. Deterministic Data Delivery – In applications with sensitive timing, a single message received later than anticipated can shut down the process, resulting in lost production or even damaged goods and equipment. Active Components – Using Ethernet for control increases the number of failure points in a system due to the need for active components. Switches, repeater and hubs are active devices, containing complex digital circuitry and requiring power (AC in most cases) to operate. The failure of a switch or hub will effectively cause a communications failure for all of the devices attached to that device’s ports, including other hubs or switches that may be attached to one or more ports of the failed device. The devices attached to the failed hub or switch will be unable to communicate with the rest of the plant network until the switch is replaced or repaired. Security - It is essential that the factory control network be isolated from the corporate IT network and that unauthorized access be prohibited. Broadcast storms, network upgrades, and activities by IT personnel can also severally impact the operation of the control network. Lack of an application layer standard – Ethernet technology provides a set of physical media definitions, a scheme for sharing that physical media and a simple frame format and addressing scheme for moving packets of data between devices. The TCP/IP protocol suite provides a set of services that two devices may use to communicate with each other over an Ethernet LAN. However, TCP/IP does not guarantee effective communicate or interoperability. A standard application layer is a necessity for universal interoperability over Ethernet- TCP/IP. Ethernet as a Control Network Page 14 of 16 INDCOMM 2003
  15. 15. Ethernet as a Control Network One of the most common arguments that traditionally has been used against the use of Ethernet for control is that Ethernet is non-deterministic. Determinism enables users to accurately predict data transmission and guarantee its arrival at the same time every time (or to quickly recognize that it did not arrive and take appropriate action). The improvements in Ethernet technology mentioned earlier in this article have improved the determinism and performance of Ethernet to a great extent. Switches break up collision domains into single devices or small groups of devices, effectively reducing the number of collisions to almost zero. CSMA/CD provides the collision mechanism for detecting and recovering from contention for the network when it does occur. Furthermore, there are efforts in place to create a prioritization scheme for messages over Ethernet (IEEE 802.1p) that if implemented inside switches could potentially be used to prioritize control/alarm message packets over programming/data packets on Ethernet. However, all of these are untried technologies in high-speed control applications. In many applications with sensitive timing, a single message received later than anticipated can shut down the process, resulting in lost production or even damaged goods and equipment. Variable packet latency or dropped packets within Ethernet switches could potentially cause this to happen. Losing a hub or switch in an information-only application will result in lost production data; losing one in a control application can result in lost production and possible damage to the production equipment itself. These and other issues must be resolved in order to rationally determine the types of control applications for which Ethernet- TCP/IP technology is a good or even an acceptable solution. Security can be implemented by providing a “firewall,” which denies access to anyone who does not have an authorized IP address to access the network. Special security software is also available. Because broadcast storms (excessive transmission of broadcast traffic), network upgrades, and other activities by IT personnel can several impact the operation of the control network, IT personnel must be well informed and trained in the special requirements of control networks. Most importantly, the lack of a standard application layer has been partly resolved with the introduction of industrial automation oriented protocols like EtherNet/IP, IDA, ProfiNet, etc. Ethernet as a Control Network Page 15 of 16 INDCOMM 2003
  16. 16. Conclusion The global acceptance of Ethernet-TCP/IP has made it a popular choice for many end users and for a wide variety of network applications. It offers an abundance of compatible products and a high data throughput at a relatively low cost. As end users begin looking to expand Ethernet’s responsibilities on the plant floor, they should consider the following: • Can the number and types of devices in the system, the frequency of data exchanges, and the sizes and types of data packets on the wire be managed in order to deliver an acceptable level of performance and determinism for the application? • Can information and control messages be successfully mixed on the same network? • Is there a reliable source of power available for all active media components? • Will the application be adversely affected by Ethernet’s complex rules regarding cable lengths and configuration, and the increased number of failure points in a system due to the need for active components such as hubs and/or switches? • Will the devices required to solve the application interoperate, or does each vendor use their own application level protocol? • Does the application require fully redundant media? • Do all of the devices and media components meet the environmental specifications and agency approvals required for the application (such as temperature, humidity and vibration)? • Who will install, manage and maintain the Ethernet network? • Is the plant floor environment electrically noisy, and how will that impact the performance of the Ethernet network? • Is the cost per connection point acceptable for the application? Many of these questions are common to all industrial automation networks and are not unique to Ethernet alone, but Ethernet-based solutions must address them. Ethernet has a bright future in industrial automation applications. However, care must be taken that it is carefully applied to applications for which its features and limitations are a good match. Ethernet as a Control Network Page 16 of 16 INDCOMM 2003